1. Field of the Invention
The present invention concerns the area of surface coatings, the said coatings being in the form of organic films. It relates specifically to a method for forming copolymer organic films by electrochemical grafting on electrically conductive or semi-conductive surfaces using aqueous solutions of precursors that are appropriately selected in order to permit the formation of these organic films in a simple and reproducible manner, to the surfaces obtained by using this method, and to their applications, in particular for preparation of microelectronic components, biomedical devices or screening kits, and to kits for implementing said method.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
At the present time, several techniques are available to form thin organic films on substrates, each based on a family or class of adapted molecules.
The coating method using centrifugation known as “spin coating” or the relative methods of immersion (dip coating) or vaporization (spray coating) do not require any specific affinity between the deposited molecules and the substrate of interest. In fact, the cohesion of the deposited film is essentially based on the interaction between the constituents of the film, which may be, e.g., cross-linked after being deposited to improve stability. These techniques are very versatile, applicable to all types of surfaces to be covered, and are very reproducible. However, they do not permit any effective grafting between the film and the substrate (it is a case of simple physisorption), and the thicknesses produced are difficult to control, especially for the thinnest deposits (less than 20 nanometers). In addition, the spin coating techniques do not allow uniform deposits unless the surface to be covered is essentially flat. The quality of the films obtained by “spray coating” is dependent on the wetting of the surfaces by the spray, since the deposit does not essentially become film-like until the drops coalesce. Thus, for a given polymer, generally only one or two organic solvents exist that are capable of yielding satisfying results in terms of controlling the thickness and the homogeneity of the deposit.
Other techniques for forming an organic coating on the surface of a support, such as plasma deposition, which is described in the articles by M. Konuma, “Film deposition by plasma techniques,” (1992) Springer Verlag, Berlin; H. Biederman and Y. Osada, “Plasma polymerization processes,” 1992, Elsevier, Amsterdam, or photochemical activation are based on a similar principle: producing unstable derivatives of a given precursor close to the surface to be covered which evolve by forming a film on the substrate. While plasma deposition does not require any specific properties of these precursors, photo-activation requires the use of photosensitive precursors, the structure of which evolves under the influence of light. These techniques generally give rise to the formation of adherent films, even though it is usually impossible to discern whether this adhesion is due to cross-linking of a topologically closed film around the object or a real formation of interface bonds.
Self-assembly of organic monolayers is a very simple technique to use (A. Ulman, “An introduction to ultrathin organic films from Langmuir-Blodgett films to self-assembly,” 1991, Boston, Academic Press). Still, this technique requires the use of molecular precursors having an adequate affinity for the surface of interest to be coated. Then precursor-surface pairs can be spoken of, such as the sulfur compounds that have an affinity for gold or silver, the tri-halogen silanes for oxides like silica or aluminium, the polyaromatics for graphite or carbon nanotubes. In all cases, the film formation is based on a specific chemical reaction between a part of the molecular precursor (the sulfur atom in the case of thiols, for example) and some “receptor” sites on the surface. A chemisorption reaction ensures bonding. Thus, at ambient temperature and in solution, films with molecular thickness (less than 10 nm) are obtained. However, while the pairs involving oxide surfaces give rise to the formation of films that are very tightly grafted (the Si—O bond involved in the chemisorption of the tri-halogen silanes on silica is among the most stable in chemistry), this is of no use when there is an interest in metals or semi-conductors without oxide. In this case, the interfacial bond between the conductive surface and the monomolecular film is fragile. Thus, self-assembled monolayers of thiols on gold desorb when heated above 60° C. or in the presence of a good solvent at ambient temperature, or even when they come in contact with an oxidizing or reducing liquid medium. In a similar manner, the Si—O—Si bonds are weakened when they are in an aqueous medium, i.e., humidity, especially with the effect of heat.
The electrografting of polymers is a technique based upon the initiation and the polymerization by propagation of an electrically induced chain of electroactive monomers on the surface of interest, playing simultaneously the role of electrode and that of a polymerization primer (S. Palacin et al., “Molecule-to-metal bonds: electrografting polymers on conducting surfaces,” Chem Phys Chem, 2004, 10, 1468). Electrografting requires the use of precursors that are adapted to its mechanism of initiation by reduction and of propagation, generally anionic, since cathodic electrografting, which is applicable on noble metals and non-noble metals, is often preferred to anodic electrografting (which is only applicable to noble metals). “Depleted vinyl” molecules, i.e., vinylic moieties bearing electro-attractor functional groups like the acrylonitriles, the acrylates, the vinyl pyridines, etc., are particularly adapted to this method, which gives rise to a number of applications in the area of microelectronics and in the biomedical area. The adherence of the electrografted films is ensured by a carbon-metal type covalent bond (G. Deniau et al., “Carbon-to-metal bonds: electrochemical reduction of 2 butenenitrile,” Surface Science, 2006, 600, 675-684).
According to this electrografting technique, polymerization is essential for the formation of the carbon/metal interface bond: it has actually been shown (G. Deniau et al., “Coupled chemistry revisited in the tentative cathodic electropolymerization of 2-butenenitrile,” Journal of Electroanalytical Chemistry, 1998, 451, 145-161) that the electrografting mechanism proceeds via an electroreduction of the monomers on the surface to yield an unstable radical anion, which, if not close to other polymerizable molecules, desorbs back to the solution (op. cit.). Besides the desorption reaction, the addition reaction (of the Michael addition type) of the first chemisorbed radical anion on a free monomer, offers a stabilizing route for the transient species: indeed the product of this addition yields a new radical anion which charge is now “at a distance from” the surface, which contributes to stabilizing the adsorbed structure. That dimeric radical anion itself may again be added to another free monomer etc.: each new addition gives additional stability by relaxation of the charge/polarized surface repulsion, which means that the interface bond of the first, temporarily, radical anion becomes more and more stable as the polymerization extents. In other words, it has been asserted that a vinylic monomer that cannot be polymerized cannot be electro-grafted.
Among the different techniques mentioned above, electrografting is the only technique that makes it possible to produce grafted films with a specific control of the interface bond. In addition, in contrast to the plasma or photo-induced techniques, electrografting does not generate reactive species except in the immediate vicinity of the surface of interest (in the double electrochemical layer, the thickness of which is of several nanometers in the majority of cases).
It seems now admitted that obtaining grafted polymer films by electrografting of activated vinyl monomers on electrically conductive surfaces proceeds via an electro-initiation of the polymerization reaction starting from the surface, followed by a growth in the chains, monomer by monomer. The reaction mechanism of electrografting has been described, in particular, in the articles of C. Bureau et al., Macromolecules, 1997, 30, 333; C. Bureau and J. Delhalle, Journal of Surface Analysis, 1999, 6(2), 159 and C. Bureau et al., Journal of Adhesion, 1996, 58, 101.
By way of example, the electrografting mechanism of acrylonitrile by cathodic polarization can be represented by Diagram A following:

In this diagram, the grafting reaction corresponds to step 1, where the growth takes place starting from the surface. Step 2 is the principal parasitic reaction, which leads to a non-grafted polymer. This reaction is limited by the use of high monomer concentrations.
The growth of the grafted chains is thus carried out by purely chemical polymerization, i.e., independently of the polarization of the conductive surface that has given rise to the grafting. This step is thus sensitive to (and is in particular interrupted by) the presence of chemical inhibitors of the growth, in particular by protons.
In Diagram A above, where electrografting of acrylonitrile using cathodic polarization is considered, the growth of the grafted chains is carried out by anionic polymerization. This growth is interrupted, in particular, by protons and it has also been demonstrated that the amount of protons is the major parameter that control the formation of polymers in solution. Information obtained in the course of synthesis, especially the shape of voltamograms that accompany the synthesis, (see in particular the article by C. Bureau, Journal of Electroanalytical Chemistry, 1999, 479, 43) show clearly that traces of water, and more generally labile protons of protonic solvents, are sources of protons that are detrimental to the growth of the grafted chains.
On the whole, if ways to produce chemical bonds on electrically conductive or semi-conductive substrates by electrografting of different precursors using organic solutions are known, it remains difficult to obtain electrografted films from aqueous solutions, since the corresponding mechanism (anionic-type polymerization) prohibits working in water. Up to the present time, only aryl diazonium salts have made it possible to approach a solution to this problem.
Indeed, as described, e.g., in the French patent application FR-A-2 804 973, electrografting of precursors such as aryl diazonium salts, that carry a positive charge, can be carried out due to a cleavage reaction after reduction of the cation, to yield a radical that chemisorbs on the surface. Just as in electrografting of polymers, the electrografting reaction of aryl diazonium salts is electro-initiated and leads to the formation of interface chemical bonds. In contrast, to the electrografting reactions of vinylic polymers, the electrografting of aryl diazonium salts does not require a coupled chemical reaction to stabilize the chemisorbed species formed by charge transfer since this species is electrically neutral and not charged negatively as in the case of a vinylic monomer. It thus leads—a priori—to a stable surface/aryl group adduct.
However, it has been shown, especially in the French patent application FR-A-2 829 046, that aryl diazonium salts lead to thin organic films that can grow on themselves: once the grafting on the initial surface has been carried out by electro-cleaving and chemisorption reaction, the film grows by polarisation-induced reaction as conducting polymers but at the cathode. This results in difficulties in controlling the organic film thicknesses resulting from electrografting of aryl diazonium salts. On the other hand, the combination in aqueous solution of a diazonium salt and a vinylic monomer can lead to the formation of a grafted film under the condition that the vinylic monomer is soluble in water. However, this method of obtaining grafted films is limited to the rare vinylic monomers that are soluble in water like some acrylic acids, some hydroxylated or aminated vinylic monomers and generally leads to poor quality films (see article by Bell and Zhang, Journal of Applied Polymer Science, 1999, 73, 2265-272).
The electro-grafting reactions that are currently available according to the prior art thus make it possible to easily obtain a certain variety of organic films on different conductive and semi-conductive substrates using organic solutions. Nevertheless, it will be necessary to extend this range in order to respond to the demand of industry, to diversify the usage properties of such materials and thus their application possibilities. In addition, the protocols used in industry make use of organic solvents that are notoriously toxic and costly. Thus, it is also desirable to propose new process that are less polluting and more profitable for industries.